It has been shown that the partial molar excess Gibbs energy (G̅ex) can be used as an indicator of hydrophobicity and hence contributes to explaining the behaviour of collectors in the recovery of minerals by flotation. This is because hydrophobicity of a mineral particle on which a collector is adsorbed is related to the fact that the alkyl chain associated with the collector behaves in the same way as any hydrocarbon would in a non-ideal mixture with water. Such mixtures are quantified thermodynamically by determining the activity coefficient of the hydrocarbon moiety of the collector and hence its G̅ex in the mixture. The present paper is aimed at extending the above concepts in two different ways. Firstly, an investigation is made of the relationship between G̅ex and contact angle and/or recovery for systems such as ionic liquids and aminothiophenols. Secondly, on the basis of the observation that increases in G̅ex are indicative of increases in the hydrophobicity of the mineral particle-collector aggregate, a study was undertaken to examine, respectively, the effects of the presence of electrolytes or alcohols in the system, of pH and changing concentrations of the collector, on G̅ex of the collector alkyl group and hence its hydrophobicity. In the first instance, the relationship between G̅ex and contact angle and recovery is sustained. With respect to the latter, the absence of any correlation between G̅ex and recoveries supported the proposition that the major effect of electrolytes and added alcohols is rather on particle – bubble attachment. Similarly, it is also indicated that since the change in G̅ex is minimal when collector concentration or pulp temperature is increased these changes do not significantly affect the hydrophobicity of the collector-mineral particle and hence also the recovery in a microflotation.